The present invention generally relates to optical submarine telecommunication systems and more particularly to a power switching circuit for use in a submarine cable branching unit of a submarine cable system that achieves telecommunication between three or more cable landing stations.
In optical submarine cable systems that achieve telecommunication between three or more cable landing stations, a submarine branching unit is employed for connecting optical cables connected to respective cable landing stations and for feeding electric power to the repeaters in the cables as indicated in FIG. 1A.
Referring to FIG. 1A showing cable landing stations A, B and C connected with each other by respective optical cables at a submarine branching unit BU, it will be noted that each of the optical cables includes a number of repeaters REP fed by a power feed line provided in the optical cable. Thus, the repeaters REP as well as the submarine branching unit BU itself are supplied with electric power from the corresponding cable landing stations by way of feeding path formed by the power line in the optical cables. In FIG. 1A, as well as in FIGS. 1B and 1C, the power line in the optical cable is represented by a continuous line.
Typically, the feeding of the electric power is achieved bilaterally from two cable landing stations via respective optical cables connected at the submarine branching unit BU. Alternatively, the electric power is fed unilaterally from one of the cable landing stations to the submarine branching unit BU that is grounded to the sea floor, along an optical cable that forms a single power feed path.
When using a submarine branching unit that merges three communication paths or branches, two of the cable landing stations are used for the bilateral feeding of electric power and the remaining one cable landing station is used for the unilateral feeding as indicated in FIG. 1B. Further, such a submarine branching unit is constructed, as proposed in the Japanese Patent Application 2-182150, to maintain communication between two cable landing stations via two optical cables, even in the event that there occurs a failure in any one of the three optical cables.
FIG. 1C shows the occurrence of such a failure in the system of FIG. 1B, wherein it will be noted that the submarine branching unit BU is constructed to achieve a bilateral power feeding between the stations A and C and further to achieve a unilateral power feeding from the station B to the ground or sea floor as indicated by SE.
Referring to FIG. 1C showing an example in which a failure has developed in the optical cable from the station A to the submarine branching unit BU, the submarine branching unit BU switches the power feed path such that the feeding of electric power is still maintained by setting a bilateral power feed path between the stations B and C and further setting a unilateral power feed path for the station A. Thereby, the bilateral power feed path between the stations B and C are maintained active, even when the unilateral power feed path from the station A is lost due to the failure in the optical cable from the station A to the submarine branching unit BU.
FIGS. 2A and 2B show a conventional power switching circuit used in the submarine branching unit BU, wherein it will be noted that the branching unit BU achieves a three-way branching of the telecommunication paths similarly to the submarine branching unit BU described with reference to FIGS. 1A-1C.
Referring to FIG. 2A, it will be noted that each of the three branches includes a relay that passes a current in a direction from a central node N to the corresponding cable landing station as indicated by arrows. For example, the branch extending from the node N to the station C includes a relay RL1, the branch extending from the node N to the station B includes a relay RL2, and the branch extending from the node N to the station A includes a relay RL3, wherein all of the relays RL1-RL3 are energized only when the current flows through respective branches in the direction indicated by arrows and cooperate with respective switch circuits rc.sub.1, rc.sub.2 and rc.sub.3. Here, it should be noted that the switch circuits are urged to close the respective branches and simultaneously open the respective ground paths in the inactivated or non-energized state of the relays.
For example, the switch circuit rc.sub.1 cooperates with the solenoid of the relay RL1 provided in the branch extending from the central node N to the station C, and closes the current path between the node N and the station B, provided that the relay RL1 is not energized. Thereby, the ground path between the station B and the sea floor on which the branching unit BU is opened. Upon energization of the relay RL1, on the other hand, the switch circuit rc.sub.1 is urged to open the current path between the node N and the station B, and the foregoing ground path from the station B to the sea floor is closed by the switch circuit rc.sub.1. Similarly, the switch circuits rc.sub.2 and rc.sub.3 cooperate with the solenoids of the relays RL2 and RL3 provided in the branches extending from the central node N to the station B and from the central node N to the station A respectively, and close the respective current paths from the node N to the station A and from the node N to the station C in the non-energized state of the relays RL2 and RL3. Thereby, the switch circuits rc.sub.2 and rc.sub.3 also open the ground path from the station A or station C to the sea floor. Upon energization of the relays RL2 and RL3, on the other hand, the switch circuits rc.sub.2 and rc.sub.3 are urged to open the foregoing feed paths between the station A and the node N and between the station C and the node N, and the feed path from the station A as well as the feed path from the station C are grounded via the switch circuits rc.sub.2 and rc.sub.3 thus actuated. In FIG. 2A, it should be noted that the switches rc.sub.1 -rc.sub.3 are represented in the non-energized state of the respective relays RL1-RL3.
It should be noted that each of the switch circuits includes a normally-open contact NO, a normally-closed contact NC and a common terminal COM, wherein the common terminal COM, being connected to the end of the optical fiber cable extending to the corresponding station, is connected to the normally-closed contact NC in the deactivated state of the relay. In the case of the relay RL1, for example, the common terminal COM of the switch circuit rc.sub.1 at the end of the cable from the station B, is connected to the normally-closed contact NC when the relay RL1 is not energized or energized in the direction opposite to the arrow. When energized in the direction of the arrow, on the other hand, the relay RL1 causes the switch circuit rc.sub.1 to switch the connection of the common terminal COM from the normally-closed contact NC to the normally-opened contact NO. In FIG. 2A, it should be noted that the continuous lines of the switch circuits rc.sub.1 -rc.sub.3 represent the state in which no energization is applied to the relays RL1-RL3. On the other hand, the broken lines indicate the state in which the relays RL1-RL3 are energized. As already noted, the relays RL1-RL3 are energized only when the drive current flows in the direction of the arrows.
Thus, when a bilateral feed is achieved from the station A to the station C, it will be noted that the relay RL3 in the bilateral current path from the station A to the station C is not energized in view of the opposite direction of the drive current, while the relay RL1 is energized. The relay RL2 is not included in the current path and is not activated either. See the state of FIG. 2B, wherein the hatched box represents the energized state of the relay. Thereby, the switch circuit rc.sub.1 is activated to connect the cable from the station B to the ground provided by the sea floor, and there is formed a unilateral feed path in addition to the bilateral feed path. As a result of the energization of the relay RL1 as such, it should be noted that the common terminal COM of the switch circuit rc.sub.1 is now contacted to the normally-open contact NO and the cable extending from the station B is connected to the sea floor ground. It should be noted that the switch circuits rc.sub.2 and rc.sub.3 of the relays RL2 and RL3 sustain the bilateral current path in view of the non-energized state of the relays RL2 and RL3.
When there occurs a failure in the path connecting between the stations A and C in the state of FIG. 2B, on the other hand, the relay RL1, hitherto being energized by the drive current through the bilateral feed path, is now deenergized due to the interruption of the drive current, and the switch circuit rc.sub.1 of the relay RL1 inevitably recovers the original state shown in FIG. 2B by a broken line in which the common terminal COM is connected to the normally-closed terminal NC. Thereby, it should be noted that the unilateral feed path between the station B and the sea floor is also disconnected. In other words, the power supply to the branching unit BU is totally interrupted and the branching unit BU is lost from the system.
Thus, in order to maintain the unilateral feed path and to save the branching unit BU, there is provided a self-sustaining relay RL4 in the ground path, with a corresponding switch circuit rc.sub.4 such that a normally-open switch circuit rc.sub.4-1 of the switch circuit rc.sub.4 is provided parallel to the normally-open contact of the switch circuit rc.sub.1 for bypassing the same and such that a normally-closed switch circuit rc.sub.4-2 is connected in series to the relay RL2.
In the foregoing construction, it should be noted that the self-sustaining relay RL4 is energized in response to the unilateral feeding of the drive current that flows from the sea floor to the station B, such that the switch circuit rc.sub.4-1 forms a self-sustaining current path that preserves the unilateral feeding path to the ground even after the relay RL1 has returned to the original, non-energized state. Of course, it is possible to provide a similar self-sustaining ground circuit to other grounded feed paths connected to the station A or C.
FIGS. 3A-3C show the state of the branching unit BU and the voltage distribution between the stations, wherein FIG. 3A shows the state of the branching unit BU, while FIG. 3B shows the voltage distribution between the stations A and C along the bilateral feed path. Further, FIG. 3C shows the voltage distribution in the event there occurred a failure in the path from the node N to the station C. It should be noted that FIG. 3A corresponds to FIG. 2B and represents a normal state in which a bilateral feeding is achieved between the stations A and C and simultaneously a unilateral feeding is achieved between the station B and the sea floor.
In the normal state of the branching unit BU shown in FIG. 3A, it will be noted that a positive voltage is supplied to the station A and a negative voltage to the station C, and the voltage on the cable changes generally linearly from the station A to the station C as indicated in FIG. 3B.
When a failure has developed between the branching unit BU and the station C as indicated by a cross and the cable is grounded as indicated in FIG. 3C as a result of such a failure, the voltage level of the cable becomes zero at the portion of the cable where the failure has occurred. Thereby, the electric charges accumulated in the cable are released suddenly in the vicinity of the site of failure, and a large current pulse tends to flow in the direction opposite to the direction of the current supplied for feeding power.
FIG. 4A shows such a flow of the current associated with such a sudden release of the electric charges on the optical cable, wherein the vertical axis of FIG. 4A shows the voltage of the power feed path while the horizontal axis represents the time. Further, FIGS. 4B and 4C show the mechanism of such a current pulse in the form of equivalent circuit diagram. FIG. 4B shows the normal state where no failure has occurred. In this state, the electric charges are accumulated on the insulating shield or other insulating part of the optical cables that acts as a capacitor while the feed current is being supplied as indicated by an arrow in FIG. 4B. When there occurs a short circuit, on the other hand, the electric charges in the capacitor are suddenly discharged as indicated by arrows in FIG. 4C, and there appears a large reverse voltage pulse shown in FIG. 4A in response to such a failure.
Such a large reverse voltage pulse causes a large reverse current to flow through the cable, while the large reverse current in turn causes an unwanted energization of the relay RL3. In the normal state of FIG. 3A, it should be noted that the direction of the current flowing from the station A to the station C has been opposite to the direction of the current that energizes the relay RL3, and the switching of the switch circuit rc.sub.3 has not occurred. On the other hand, when there flows a large reverse current through the relay RL3, there is a substantial risk that the relay RL3 is energized and the switch circuit rc.sub.3 switches.
It should be noted that such a switching of the switch circuit rc.sub.3 causes the grounding of the reverse current, and the energization of the relay RL3 disappears immediately. In response to the disappearance of energization of the relay RL3, the switch circuit rc.sub.3 resumes the original state and the cable extending from the station C is connected to the feeding path that includes the relay RL3. Thereby, the relay RL3 is re-energized and the switch circuit rc.sub.3 causes switching. Further, such on-off operations of the relay RL3 and hence on-off switching of the switch circuit rc.sub.3 are repeated a plurality of times to cause a chattering. As high voltage is applied to the switch circuit rc.sub.3, there is a substantial risk that the switch circuit rc.sub.3 is seriously damaged as a result of the chattering. It should be noted that the relays RL1-RL3 are typically formed of vacuum relay. In the submarine cable systems where high reliability is absolute requirement, such a chattering is not tolerated and has to be eliminated.